WO-A-98/06064 describes post-processing of a computer-generated image of a three-dimensional object for display as a pixel image. A focus depth is specified either by the user or from within the system for each image frame. The image pixels are grouped in non-overlapping blocks of adjoining image pixels with a single respective depth value pre-specified or calculated at run-time for each block. Each block is extracted in turn from the image, and the pixels within a block are filtered with a common set of filter coefficients. Padding may be applied to fill areas where a full filter kernel cannot be formed. An example of padding is repeating pixel edge values or “wrapping round” values from the opposite edge of the block in order to provide sufficient values of the right general order to construct a filter kernel centered on these pixels in the boundary area.